Transcriptome analyses have suggested that, although only 1-2% of the mammalian genome is protein-coding, 70-90% is transcriptionally active (Carninci et al., 2005; Kapranov et al., 2007; Mercer et al., 2009). Ranging from 100 nt to >100 kb, these transcripts are largely unknown in function, may originate within or between genes, and may be conserved and developmentally regulated (Kapranov et al., 2007; Guttman et al., 2009). Recent discoveries argue that a subset of these transcripts play crucial roles in epigenetic regulation. For example, genes in the human HOX-D locus are regulated in trans by HOTAIR RNA, produced by the unlinked HOX-C locus (Rinn et al., 2007), and during X-chromosome inactivation, Tsix, RepA, and Xist RNAs target a chromatin modifier in cis to control chromosome-wide silencing (Zhao et al., 2008). Interestingly, all four RNAs bind and regulate Polycomb Repressive Complex 2 (PRC2), the complex that catalyzes trimethylation of histone H3-lysine27 (H3-K27me3) (Schwartz and Pirrotta, 2008). These observations support the idea that long ncRNAs are ideal for targeting chromatin modifiers to specific alleles or unique locations in the genome (Lee, 2009) (Lee, 2010).
RNA-mediated recruitment is especially attractive for Polycomb proteins. First identified in Drosophila as homeotic regulators, Polycomb proteins are conserved from flies to mammals and control many aspects of development (Ringrose and Paro, 2004; Boyer et al., 2006; Lee et al., 2006; Schuettengruber et al., 2007; Pietersen and van Lohuizen, 2008; Schwartz and Pirrotta, 2008). Mammalian PRC2 contains four core subunits, Eed, Suz12, RbAp48, and the catalytic Ezh2. In humans, aberrant PRC2 expression is linked to cancer and disease (Sparmann and van Lohuizen, 2006; Bernardi and Pandolfi, 2007; Miremadi et al., 2007; Rajasekhar and Begemann, 2007; Simon and Lange, 2008). Despite growing recognition of Polycomb's role in health, little is known about their regulation in vivo. In flies, Polycomb complexes may contain sequence-specific DNA-binding factors, such as Zeste, Pipsqueak (PSQ), or Pho, to help bind Polycomb-response elements (PRE) (Ringrose and Paro, 2004; Schwartz and Pirrotta, 2008). By contrast, mammalian Polycomb complexes are not thought to contain such subunits. Therefore, their mechanism of recruitment to thousands of genomic locations remains poorly understood, though PRE-like elements (Sing et al., 2009; Woo et al., 2010) and Jarid2 may facilitate binding (Li et al.; Pasini et al.; Peng et al., 2009; Shen et al., 2009). Interestingly, several PRC2 subunits have potential RNA-binding motifs (Denisenko et al., 1998; Bernstein and Allis, 2005; Bernstein et al., 2006b)—a possibility borne out by postulated functional interactions between Tsix/RepA/Xist RNA and PRC2 for X-inactivation (Zhao et al., 2008) and by HOTAIR and PRC2 for HOX regulation (Rinn et al., 2007). Recent work also identified several short RNAs of 50-200 nt as candidate PRC2 regulators (Kanhere et al., 2010). Control of Polycomb repressive comp lex 1 (PRC1) may also involve RNA (Yap et al., 2010).
In spite of their ubiquity, the structure and function of many long ncRNAs remain largely uncharacterized. Recent studies suggest that some long ncRNAs may have a function as an epigenetic regulator/RNA cofactor in chromatin remodeling and tumor suppression. Although knockdown technologies employing siRNAs and shRNAs have become staples in functional analysis of microRNAs (miRNAs) and cytoplasmically localized messenger RNAs (mRNAs) (4-6), these methods have been reported in some instances to be less consistently effective for long ncRNAs localized to the nucleus (Jepsen et al., Oligonucleotides, 14, 130-146 (2004)).